The present invention, in some embodiments thereof, relates to gaseous fluids in general, and more particularly, but not exclusively, to a system and method for transporting and storing gas hydrates.
For decades, storage and transport of natural gas has been problematic and expensive, preventing the exploitation of many small and medium-sized natural gas fields. Generally, the gas is transported over pipelines to a processing plant where the gas is liquefied and stored as liquefied natural gas (LNG) or compressed and stored as compressed natural gas (CNG). Distribution of the LNG and CNG from the processing plant is then generally done by sea vessels and/or land vehicles specially adapted to contain the gas in its respective form.
In an attempt to overcome the high costs and transportation difficulties associated with natural gas transport and storage and promote the exploitation of small and medium-sized natural gas fields, a relatively recent trend is to promote the use of clathrates technology. This involves converting the natural gas into natural gas hydrates (NGH) which may be processed as hydrate slurry or further processed into other forms, including hydrate pellets, and may provide an economical option for both storing and transporting natural gas and other gases as an alternative to liquefying or compression.
Clathrates are non-stoichiometric crystalline compounds consisting of at least two molecular species, where one species physically entraps the others within a cage-like structure. The species forming the cage-like structure is commonly referred to as the host, while the caged component is commonly referred to as the guest. When the cage-like structure is made up of water molecules bonded together, the crystalline compounds formed are known as clathrate hydrates or gas hydrates.
In gas hydrates, the host-lattice is created by water molecules connected together through hydrogen bonding. The guest molecule is held in place inside cavities of the hydrogen-bonded water molecules, and the lattice is stabilized by van der Weals forces between host and guest molecules without chemical bonding between the host-lattice and guest molecule. The host-lattice is thermodynamically unstable without the presence of a guest molecule in the cavity, and without the support of the trapped molecules, the lattice structure of gas hydrates will collapse into conventional ice crystal structures or liquid water. Most low molecular weight gases, including O2, H2, N2, CO2, CH4, H2S, Ar, Kr, and Xe as well as some higher hydrocarbons and freons, will form hydrates at suitable temperatures and pressures.
Use of NGH as a substitute for LNG and CNG generally involves three stages; production, transportation, and regasification. Some examples of systems and methods for producing gas hydrates and gas hydrate slurry and for regasification are disclosed in US Patent Application Publication No. US 2011/0217210 to Katoh et al., WIPO International Publication WO 2015/087268 to Sangwai, U.S. Pat. No. 8,334,418 to Osegovic et al., and U.S. Pat. No. 8,354,565 to Brown et al. Some examples of systems and methods for transporting the gas hydrate in marine vessels are disclosed in “Frozen Hydrate for Transport of Natural Gas”, Gudmundsson, J. S. and Borrehaug, A., Proceedings, 2nd International Conference Natural Gas Hydrates, Jun. 2-6, 1996, Toulouse, pp. 415-422; Japanese Patent Application No. 2004-070249, “Gas-Hydrate Transportation Vessel”, to Ichiji et al.; and Japanese Patent Application No. 2002-089098, “Gas Hydrate Pellet Transport Ship”, to Ichiji et al.